1. Field of the Invention
The present invention relates to a liquid crystal display device driven by an alternating current, especially to a power source apparatus which is used for driving a liquid crystal display device and has four intermediate potentials between a maximum potential and a minimum potential.
2. Description of the Related Art
A liquid crystal display device has an arrangement of a plurality of display dots, which are formed of cross points of a group of segment electrodes and a group of common electrodes. The liquid crystals between the two groups of electrodes are controlled in their crystal arrangement by means of potential difference between the groups of segment electrodes and the group of common electrodes in order to display images.
Such a liquid crystal display device uses a dynamic driving control in which potentials set between the segment electrodes and the common electrodes are generally reversed in polarity for each frame. For such a dynamic driving control, there are generally prepared power sources for providing four intermediate potential levels in addition to a maximum potential power source and a minimum potential power source, the intermediate potential levels being obtained by dividing the potential difference between the maximum potential level and the minimum potential level into four parts. A power source having a suitable potential level is selected in accordance with a display data, and the voltage obtained by the selected potential power source is delivered and applied to the group of segment electrodes and the group of common electrodes.
The four intermediate potential levels between the potential levels set by the maximum potential power source and the minimum potential power source are generated by a voltage dividing circuit which is formed by a combination of resistors.
FIG. 8 shows a conventionally known power source circuit for driving a liquid crystal display device. This power source circuit is arranged such that any one of terminals V1, V2, and V3 supplies, corresponding to frame signal FR, a power source output having a predetermined potential for A.C.-driving a liquid crystal display device.
For instance, as apparent from the relationship shown in FIG. 9 and the timing chart shown in FIG. 12, the maximum potential level "V", the two intermediate levels "V2H" and "V1H", and the minimum potential level "0" are put out when frame signal FR is "0", whereas, when frame signal FR is "1", the maximum potential level "V", the two intermediate potential levels "V3L" and "V2L", and the minimum potential level "0" are put out. These potential level combinations are alternately generated whenever the frame signal changes between "0" and "1". These potential signals are supplied through the segment output level selecting circuit shown in FIG. 10 and the common output level selecting circuit shown in FIG. 11 to the segment electrodes and the common electrodes of the liquid crystal display device.
FIG. 8 shows a 1/5 pre-biased power source circuit, in which resistors R1 and R4 are each set to 300 K.OMEGA., resistors R2 and R3 to 100 K.OMEGA., resistors r1 and r4 to 30 K.OMEGA., and resistors r2 and r3 to 10 K.OMEGA.. The dynamic chart shown in FIG. 12 shows a case of a 1/8 duty in which the number of common outputs is eight. The figure exemplary shows only one segment output among a plurality of segment outputs and only one common output among a plurality of common outputs.
In such a power source circuit, clock signal .phi.c formed of a pulse signal indicating a common selection signal switching timing, and lowers the output resistance of the potential dividing circuit at a moment of a common signal switching time so as to increase the response of the liquid crystals. Namely, the charging or discharging time of a capacitance provided in the liquid crystal display device is made short by connecting resistors r1-r4, which are lower in resistance value than resistors R1-R4, in parallel with resistors R1-R4 for dividing voltage V.
However, a large capacitance is required for a liquid crystal display device having a large number of display pixels. Therefore, the output resistance of the voltage dividing circuit must be made sufficiently small, or a satisfactory display quality may not be obtained. However, if the output resistance of the voltage dividing circuit is made small, the problem of increase in power consumption will occur.
FIG. 13 shows a circuit for making small the output resistance of the intermediate potential level power sources. In this circuit, operational amplifiers OP1 through OP4 are used to put out the voltages which are voltage-divided by resistors R1-R5. This circuit shows a 1/5 pre-biased case, and resistors R1-R5 are made of resistance element having a same resistance value. However, in such a circuit which uses operational amplifiers, operational amplifiers OP1-0P4 are always set in active condition, so that the power consumption at the operational amplifier portion is large.